Such arrangements have been known for some time and are exemplified by that shown in U.S. Pat. No. 3,747,577. In this publication, an apparatus is disclosed wherein two temperature-dependent resistors are arranged in different branches of a measuring bridge. The bridge is configured so that the one temperature-dependent resistor is heated to a high temperature above the temperature of that of the flowing medium because of the large current flowing therethrough. The measuring principle of this arrangement is based on the fact that the medium which flows past the resistor draws a predetermined quantity of heat away therefrom dependent upon the velocity and density of the flow. The second temperature-dependent resistor serves to compensate for the influence of the varying temperature of the flowing medium. In addition, the current flowing through this temperature-dependent resistor is selected to be so small that the temperature of the resistor is determined exclusively by the temperature of the flowing medium.
The current flowing through the bridge is controlled with the aid of a regulating device such that the heated temperature-dependent resistor exhibits a constant temperature difference with reference to the temperature of the inducted air. The current flowing through the bridge can, for example, be used as a measure for the mass per unit of time flowing past this resistor. In order to insure a fast response of the arrangement to changes in flow, the heated resistor is configured in the form of a resistor with a small thermal time constant such as a hot wire, for example. The temperature-dependent compensating resistor has resistance values which are several orders of magnitude larger than the resistance value of the hot wire. This temperature-dependent compensation resistor has a substantially larger thermal time constant because of its geometric form and its larger mass.
With this arrangement, there is the problem and disadvantage that the fast temperature changes of the flowing medium cause the arrangement to provide false measured values until the temperature of the compensation resistor is again in balance with the temperature of the flowing medium. Furthermore, under certain circumstances, as for example when the flowing medium is contaminated with solid particles, it is desirable to replace the heated temperature-dependent resistor with a resistor that is robust, the former being mechanically very sensitive because of its low mass. In this arrangement wherein the temperature of the hot wire follows the temperature of the flowing medium, the use of a massive resistor would be a great disadvantage because of a substantial reduction in the speed of response. Furthermore, because of the special dimensioning of the bridge resistors required by various peripheral conditions, only a limited use of the available heat capacity is made of which only about thirty percent is utilized for heating the hot wire. In addition, there is the necessity to increase the sensitivity of the arrangement in order to meet the growing demands on measuring accuracy of such an arrangement.
There are further developments and improvements of this type arrangement known which avoid several of the disadvantages delineated above; however, these developments create new problems and other disadvantages. In U.S. Pat. No. 4,344,322, for example, an arrangement for measuring the mass flow-rate of air in the air induction tube of an internal combustion engine is disclosed wherein both temperature-dependent resistors are configured as hot wires. For both hot wires, a temperature above the temperature of the inducted air is selected and the difference of the hot wire temperatures is held to a constant value. This arrangement has the advantage that the thermal time constants of both temperature-dependent resistors of like configuration have the same values so that temperature variations of the flowing medium cause no measuring errors based on differences in the speed of response of the resistors. However, to prevent a dependence of the measuring signal on the temperature of the flowing medium, it is necessary to place the temperature coefficients of both the total resistances of the bridge branches in a very definite relationship, the temperature-dependent resistors being arranged in the branches. This requirement can be fulfilled especially in a mass production of this arrangement only with moderate precision or with a very considerable engineering effort.
Also in these arrangements wherein only the difference of the hot wire temperatures has to be maintained at a constant value, a massive mechanically insensitive embodiment of the temperature-dependent resistors would lead to a larger response time of the bridge.
A further arrangement for measuring the mass flow rate of a flowing medium is disclosed in U.S. Pat. No. 4,283,944. In this arrangement, a temperature-dependent film resistor formed on a substrate is utilized in lieu of a hot wire. There is a flow of heat flowing between the film resistor and the substrate which leads to a reduction of the response speed of the film resistor. In order to keep this flow of heat at zero or limited to only a very low value, a further active regulated bridge circuit is needed in addition to the first active bridge for regulating the temperature of the film resistor. This bridge circuit acts exclusively to heat the substrate to the same value approximating the temperature of the film resistor. With this measure, one does provide that the temperature-dependent measuring resistor configured as a film resistor on a substrate exhibits a fast time response action which is similar to that of a hot wire; however, all the other disadvantages already mentioned above remain notwithstanding the very substantial increase in the complexity of the circuit.
Finally, it is appropriate to mention certain measures which lead to an increase in the sensitivity of the bridge circuits. Accordingly, the Wheatstone bridges utilized in the described arrangements reach their maximum selectivity when the bridge resistors of each bridge branch have the same value or, more specifically, when the bridge is symmetrical. A further doubling of the sensitivity can be obtained when two measuring resistors are utilized in mutually adjacent branches of the bridge in lieu of one measuring resistor.